On 6/3/2013 1:44 PM, Jon Elson wrote:
wrote:
If you get a 100,000 ampere lightning strike directly to anything
other than a metal building, the V=IR (voltage = current x resistance)
drop is going to raise the voltage of everything tied together many
many volts. All you can do is try to keep everything at the same
(elevated) voltage to avoid killing people/animals. This is done by
tying everything to the same local internal "ground".
This is a fantasy, unless you have 1/4 " x 3" bus bars running the
length of your house, and from the main service panel to the ground
field, which would be maybe half an acre of bare welding cable
buried in the earth.
The way it is done is all wires - power, cable, phone, antenna, dish -
enter at about same location. Entry protectors for the signal wires
connect with a short wire to the power earthing system. A surge
protector at the service panel limits the voltage of power wires to the
earthing system. In an 'event' the building "ground" can rise thousands
of volts, but all the wires rise together.
It is not perfect protection. During an 'event' a pad mounted A/C
compressor may have its ground potential at the earth where it is
located. The power system, and power wires to the compressor, can be
referenced to the earthing system, which can thousands of volts different.
A ham with a high antenna would have a better earthing system and the
common reference point may not be at the power service.
Otherwise, you simply can't get a low enough impedance to keep
the IR drop anywhere within reason. And, the induced voltages across
various points such as plumbing and plugged-in appliances can
be lethal. (later part of your post seems to agree with this.)
So, mostly, you want to try to conduct lightning strike currents
away from the building, not THROUGH the building. So, what I have
is a ground rod driven directly under the antenna, and connected
to the mast via 4 gauge cable. If the lightning current goes through
the house, it can easily set it on fire.
Jon
Suppose you have a power system earthed by a single ground rod. The
antenna entrance to the house has a ground block, as required by the NEC
and an 18 ft ground wire to the power earthing system, as required by
the NEC. You add a ground rod at the entry protector. The NEC now wants
a #6 or larger bond wire to the power earthing system. Also suppose the
ground rods have a near miraculous 10 ohms resistance to earth.
Now suppose the antenna is hit by an "average" 20,000A strike. The
potential at the rod will be 200,000V from 'absolute' earth potential.
In general 80% of the voltage drop will be in the first 3 feet (which
probably results in arcing across earth at the rod). The potential of
the antenna lead will many tens of thousands of volts from the potential
of other wiring, referenced to the power system ground rod. There will
be major damage. The #6 (and your #4) bond wire help, but not
significantly. A lightning strike is a very short event and produces
relatively high frequency current components. The inductance of the wire
is much more important than the resistance. Inductance doesn't change
real fast with larger wire. There will be arc-over from the antenna and
down leads into the house anyway.
Now suppose there is a 5,000A surge to earth on the power earthing
system. The power system 'ground' will be 50,000V above 'absolute' earth
potential. It will be tens of thousands of volts from the ground rod for
the antenna. Power wires and antenna wires will be many thousands of
volts different. That will cause damage to anything connected to both.
Without the antenna ground rod the antenna lead would be lifted with
other house wiring. The same thing happens with a strike to a tree in
the yard with one rod nearer than the other.
I am not a fan of isolated ground rods. If you really want to protect
from a direct strike you need lightning rods.
And suppose the antenna lead goes close to a CSST gas pipe, which is
bonded as the manufacturer requires. There is an arc from the antenna
lead to the CSST which melts a hole in the 0.008" wall. If you are lucky
that just ignites the escaping gas and causes a fire. (My intent is to
comment on CSST, not so much the antenna ground rod.)